This invention relates to cathode-ray tubes (CRT's) having implosion protection bands.
Cathode-ray tubes include glass envelopes that are evacuated to a very low internal pressure and accordingly, are subject to the possibility of implosion due to the stresses produced by atmospheric pressure acting on the exterior surface of the tube. This problem has been addressed in the art by providing the tubes with implosion protection bands. Such bands are used to apply a compressive force to the sidewalls of the faceplate panel of a tube to redistribute some of the faceplate panel forces. The redistribution of the faceplate panel forces decreases the probability of an implosion of the tube by minimizing tension in the corners of the faceplate panel. Implosion protection bands are also beneficial because they improve the impact resistance of the tube. Glass in compression is stronger than glass which is not in compression. The band causes compression in faceplate panel areas which otherwise are in tension. Additionally, in the event of an implosion the redistributed stresses cause the imploding glass to be directed toward the back of a cabinet in which the tube is mounted, thereby substantially reducing the probability of someone in the vicinity of the imploding tube being injured.
Implosion protection bands of the shrink fit type typically are manufactured by forming a strip of steel into a loop having the same configuration as the faceplate of the tube to be protected and by joining the two ends of the strip on one side of the loop. In some instances, the band is made by joining two identical strips on two sides of the loops. For both types of bands, the periphery of the loop is slightly smaller than the periphery of the faceplate. The loop is heated to a temperature in the range of approximately 300.degree. to 500.degree. C. and the coefficient of expansion of the material causes the loop to expand to dimensions permitting the loop to be slipped around the sidewalls of the faceplate. As the band cools it shrinks and tightly surrounds the faceplate, thereby applying the necessary implosion protection compression to the faceplate sidewalls. The compressive force can be accurately controlled by accurately dimensioning the band because the coefficient of expansion of the banding material is known.
Typically the implosion protection bands also serve the function of supporting mounting lugs which are used to mount the tube in a cabinet. The mounting lugs are permanently affixed to the implosion protection band in a convenient manner, usually welding. One type of mounting lug is a flat piece having one smooth edge. The smooth edge of the mounting lug is pressed against the outside surface of the band and the mounting lug is welded at selected points along the outside surface of the band. Problems sometime arise with this type of implosion protection band/mounting lug combination because the welding causes the formation of indentations in the band. The indentations contact the tube glass and in some instances chip the glass, thereby substantially increasing the probability of an implosion of the tube. For this reason there is a need for an implosion protection band/mounting lug structure which prevents the indentations from contacting the tube glass. The present invention fulfills this need.